1. Field of the Invention
This invention relates to antennas and, more particularly, to a practical implementation of a low-loss, broadband antenna incorporating electric and magnetic radiative components.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Electrically-small antenna elements are utilized in many low frequency (e.g., mobile communications) and high frequency (e.g., EMC testing) applications. For example, an electrically-small antenna may be used in low frequency applications to accommodate space, durability or other concerns, or in high frequency applications to achieve a particular frequency level, which may be desired for EMC testing purposes. As used herein, the term “electrically-small” refers to an antenna or antenna element with relatively small geometrical dimensions compared to the wavelengths of the electromagnetic fields they radiate. Quantitatively speaking, electrically-small antennas are generally defined as antennas which fit inside a so-called radiansphere, or a sphere with a radius, r=λ/2π, where λ is the wavelength of the radiated electromagnetic energy.
Unfortunately, electrically small antennas tend to have relatively large radiation quality factors, Q, meaning that they tend to store (on time average) much more energy than they radiate. This leads to input impedances that are predominantly reactive, which can make it difficult, if not impossible, to impedance match an electrically small antenna to an input feed over a broad range of bandwidths. Furthermore, due to the large radiation quality factor, the presence of even small resistive losses leads to very low radiation efficiencies in electrically small antennas (e.g., around 1–50% efficiency).
According to known quantitative predictions of the limits on the radiation Q of electrically small antennas, the minimum attainable radiation Q for any linearly polarized, omnidirectional antenna, which fits inside a spherical volume of radius, a, can be found by:
                    Q        =                              1            ka                    +                      1                                          k                3                            ⁢                              a                3                                                                        (                  EQ          .                                          ⁢          1                )            where k=1/λ, the wave number associated with the electromagnetic radiation. Thus, the radiation Q of an electrically small antenna may be roughly proportional to the inverse of its electrical volume (a), or inversely proportional to the antenna bandwidth. In order to achieve relatively broad bandwidth and high efficiency with a single-element, electrically small antenna of a given size, it is desirable to utilize as much of the volume (that the antenna occupies) as possible. This may be achieved, in some cases, by increasing the size of the antenna elements, while retaining an electrically-small status.
In order to achieve the fundamental limit on radiation Q given in EQ. 1, an antenna would have to excite only the Transverse Magnetic (TM01) or Transverse Electric (TE11) mode outside of the enclosing spherical surface and store no electric or magnetic energy inside the spherical surface. So while, a short linear (electric) dipole excites the TM01 mode outside of the sphere, it does not satisfy the criterion of storing no energy within the sphere, and thus, exhibits a higher radiation Q (and narrower bandwidth) than that predicted by EQ. 1.
In general, all antennas that radiate dipolar fields, such as electric and magnetic dipoles, are limited by the constraint given in EQ. 1. Though some broadband dipole designs have been successfully implemented and approach the limit given in EQ. 1, it is currently impossible to construct a linearly-polarized, omnidirectional antenna that exhibits a radiation Q less than that predicted by EQ. 1. However, while EQ. 1 represents the fundamental limit on the radiation Q for a linearly-polarized, omnidirectional antenna, it is not the global lower limit on radiation Q. For example, a compound antenna which radiates substantially equal power into the TM01 and TE11 modes can (in principle) achieve a radiation Q of approximately:
                    Q        =                              1            2                    ⁡                      [                                          2                ka                            +                              1                                                      k                    3                                    ⁢                                      a                    3                                                                        ]                                              (                  EQ          .                                          ⁢          2                )            or roughly half that of an isolated electric or magnetic dipole, which radiates the TM01 or TE11 mode, alone. In other words, the impedance bandwidth of a compound antenna can be nearly double that of an isolated electric or magnetic dipole.
Ideal compound antennas having a pair of electrically-small electric and magnetic dipoles, which are co-located and oriented to provide orthogonal dipole moments, have been theoretically and numerically examined and found to provide useful features. Such antennas are often referred to as “P×M antennas,” due to their orthogonal combination of electric (p) and magnetic (m) dipole vectors. Desirable characteristics of P×M antennas may include, but are not limited to, a useful radiation pattern (e.g., a low-gain, unidirectional radiation pattern) and a relatively broad impedance bandwidth for a given electrical size. As noted above, the radiation Q of an electrically-small P×M antenna is approximately half that of an isolated electric or magnetic dipole. Though the reduced Q should improve broadband impedance matching (at least in principle), practical implementations of P×M antennas have been problematic and have not been thoroughly investigated.
In order to provide broadband P×M operation, the dipole moments of the electric and magnetic radiators must be orthogonal in spatial orientation, substantially equal in magnitude, and in phase-quadrature over the desired operating frequency range. It is not difficult to specify the relationship between the magnitude and phase of two isolated radiators in a numerical or analytical model. In practice, however, such an antenna is usually driven from a single radio-frequency (RF) source, whose finite output impedance must be matched to the combined electric and magnetic radiator. This tends to be a particularly difficult problem due to the resonant nature of the combined electric and magnetic dipole radiator.
In some cases, a low-loss, passive feed or matching network may be used to combine the electric and magnetic radiators. However, such matching networks are often difficult to implement, due to the frequency-dependent variation in the input impedance of the two radiators. For example, variations in input impedance can make it difficult to maintain the proper magnitude and phase of the feed currents supplied to the electric and magnetic radiators. Furthermore, even when a matching network is used to combine the radiators, residual impedance mismatches may still limit the efficiency and power transfer of the antenna/matching network, and thus, the overall efficiency of the system. Although possible matching networks have been suggested, none of the currently known designs allow the combined radiator to operate efficiently over a broad range of frequencies. Therefore, the use of such designs often negates any improvements in bandwidth that may be provided by the lower radiation Q of the P×M radiator.
In principle, it should be possible to utilize electric and magnetic dipoles with complementary input impedances to provide the desired broadband operation. One such proven approach is the monopole-slot combination. This configuration is, in the ideal case, a true P×M radiator. For example, the monopole-slot antenna may be considered a two-port T-network formed with the radiation impedance of a slot antenna in the two series arms, and the radiation impedance of a monopole antenna in the shunt arm. The two-port T-network is usually terminated in a resistive load, whose value is equal to the image impedance of the T-network. However, use of a resistive load causes the antenna to have a lossy, low-pass characteristic. For this reason, the monopole-slot combination typically suffers from relatively low efficiency, even though the input impedance is more or less constant and matched. While the monopole-slot antenna is known to demonstrate a useful pattern behavior, the design is further burdened by the requirement of a ground plane.
Thus, two problems must be overcome to successfully implement a practical P×M antenna. First, practical electric and magnetic radiators must be found or designed, and second, a low-loss passive network to combine the two radiators must be implemented in such a way that P×M operation is maintained over some reasonable bandwidth. If resistive losses are to be kept to a minimum, the circulation of reactive power within the matching network must also be minimized.
As used herein, “P×M operation” is maintained when the electric and magnetic dipole moments are substantially orthogonal in spatial orientation, substantially equal in magnitude, and in phase-quadrature over a desired frequency range. In other words, the component radiators themselves must behave correctly—like electric and magnetic dipoles—so that the magnitude and phase of the far field components produced by each radiator will be in proper magnitude and phase for the superposition of the two to provide the desired performance. This enables the far field components of the electric and magnetic radiators to add up in phase.
For an isolated electrically-small electric or magnetic dipole, the above requirements are reduced to providing a matching network, which stores an opposite form of energy to that stored by the antenna. In other words, if efficiency is to be maximized, and both capacitive and inductive elements are available with the same radiation Q, a short electric dipole should be matched with an all-inductive matching network. Unfortunately, the situation is more complex with P×M antennas, since they store both electric and magnetic energy. Moreover, if the individual elements themselves are not electrically-small, each element will not store predominantly one form of energy. For example, a linear or tapered electric dipole of moderate electrical size will not store predominantly electric energy, but rather, will store both electric and magnetic energy with equipartition of energy achieved at resonance.
Thus, a need remains for a practical antenna design, which combines electric and magnetic dipole radiators to provide a low-loss, broadband implementation suitable for high-power applications.